The present invention relates to motion-imparting apparatus of a type capable of imparting motion to a load for the purpose of positioning it or for the purpose of controlling or modifying its motion. The present invention finds applications in a wide range of devices including, for example, positioning devices of manufacturing machinery. Generally, such machinery does not require the device to do mechanical work on a material, (e.g. to cut it or to deform it) but rather to control the position of a mass which moves on a low-friction bearing system. Such machines are often required to carry out a precisely-defined positioning action at a high repetition rate and with a negligible error incidence. In such a machine the use of an electromagnetic ram has advantages over other types of actuator in that it is intrinsically simple in its construction, it has a zero backlash and it has a zero control transport lag. These valuable properties allow an electromagnetic actuator to produce rapid motion with extreme precision and reliability. It is possible to apply forces of more than 20 Tonnes to be applied at several meters per second with a positioning accuracy of a few microns.
Such apparatus is also used, for example, in simulators for training or entertainment. Typically in such use a platform is moved in relation to a static structure in order to create sensations of continuous movement for human occupants of a capsule affixed to the moving platform. Such mechanisms are also used in the testing of suspension systems and stabilising apparatus, whereby the motion platform is used to create calibrated disturbance accelerations, against which the operation of the stabilising mechanism requires to be tested. The moving platform of motion-imparting apparatus is normally actuated by an array of actuator units or xe2x80x9cramsxe2x80x9d which may be driven by hydraulic fluid or gas under pressure, or by ram-like devices which are electrically operated by means of a ball-nut and screw mechanism. More recently, apparatus has been designed to use a geared-down crank fitted to rotary motor, or to employ direct electromagnetic interaction between a moving piston-shaped armature and a cylindrical stator. In the case of the latter electrical and electromagnetic machines, there is a requirement for the motion-imparting apparatus to be supported against the gravitational forces acting upon the capsule and its occupants by some means. This is important because energy would otherwise be continuously consumed within the electrical machine in order to create the thrust required to counteract the constant gravitational force acting upon the capsule. This would soon cause the motors to overheat. This effect is also encountered in other applications.
One attempt to solve this problem is described in International Patent Application published under WO93/01577. This document describes a technique that carries the load of the motion platform on a counterbalancing mechanism having a low effective spring rate. As a particular example a cranked gas spring system is described therein. Experience with mechanisms that have been constructed in accordance with the disclosure of WO93/01577 has shown that the concept of counterbalancing described therein does not provide optimal support for an electromagnetic motion base. The present invention, in one concept, is based on the realisation that a controlled spring (or a set of controlled springs) with a significant spring rate is required for optimum support.
Hitherto, a number of designs of electromagnetic actuator, or linear motor, have been produced. Various configurations of previous machines have been described in documents such as WO93/01646, which discloses an electromagnetic device arranged to operate with cylindrical symmetry as a piston-in-cylinder machine. The principal advantage of that form of construction is that the strong attractive forces between the permanent magnets within the machine and the magnetic materials that surround them are balanced about a central axis, so that the bearings of the machine do not need to withstand any large magnetic forces.
A further advantage of the cylindrical construction is that the magnetic fields of the machine are contained within the outer steel case of the actuator or ram and that they can be arranged to intersect the electrical coils of the machine with a high degree of efficiency.
Yet another advantage of the cylindrical construction is that the armature of the ram may carry a sliding seal between it and the inside surface of the stator of the ram so as to form the piston of a fluid actuator device. This is beneficial when there is a requirement to produce fast-acting electromagnetic forces that are superimposed upon or intermittent with steady or slowly-changing forces. The latter types of force are better produced by means of a fluid actuator. The electromagnetic elements would otherwise be required to consume electrical power continuously if they were themselves to provide the static or slowly-changing force.
Further, the cylindrical, piston-in-cylinder construction is appropriate to the application of the ram in of many industrial control applications where hydraulic or pneumatic rams are now employed. This is because the magnetic fields of the ram are wholly-contained within the cylindrical casing, so that the ram is tolerant to the presence of swarf or other magnetic dust that is a problem to other types of electromagnetic linear actuator.
The present invention seeks to provide a structure in which the cylindrical stator cavity of an electromagnetic actuator is divided into two parts by a seal on the piston/armature element, and includes means by which the armature assembly may also act as the force-producing element of a fluid pressure control system for example as part of a gas spring.
It is to be noted that WO93/01646 and its associated co-pending application WO93/01577 describe a ram construction for application to motion base machines in which the gas spring property of the ram is conceived to act in one direction only, so as to support the weight, that is to resist a gravitational force. For this purpose WO93/01646 describes the construction of a passage for fluid flow connection of only one part of the ram cylinder (namely the underside of the piston) to a pressurised fluid reservoir forming part of a gas spring. Both WO93/01646 and WO93/01577 describe that the upper part of the piston is allowed to vent to atmospheric pressure, directly or via an exhaust reservoir.
It is desirable to have. a more efficient method of controlling the motion of an actuator simulator mechanism than that of WO93/01577, using a combination of air pressure and electromagnetic forces in which the individual forces and force gradients of the gas springs are optimised in relation to the dimensions of the load so that the energy consumed by the mechanism is at a minimum. For this purpose the gas springs acting beneath the piston of each ram should not be designed solely for the support of the load as in WO93/01577 but rather should function as temporary reservoirs in which the potential energy resulting from the electromagnetic action of the rams is stored and from which it can be recycled a short time later. This energy recycling technique results in an economy of power consumption which improves the performance and reduces the costs of construction and operation of an electromagnetic actuator.
In one aspect, therefore, the present invention seeks to provide a motion-imparting system in which a continuous load component is supported in an especially effective manner. It is a feature of embodiments of this invention that as applied to so-called motion-bases they have a form which is easily adapted to support a variety of capsule shapes, which is physically stable and robust, which has an enhanced ability to produce large angles of pitch and roll motion and which is easily accessible for inspection and service.
According to one aspect of the present invention there is provided apparatus for imparting motion to a load comprising means for applying a perturbing force to the load and a compliant support for the load, in which there are provided means for effecting dynamic variation in the compliance of the support during operation of the apparatus.
In one embodiment of the invention the force-applying means is an electromagnetic actuator. For many applications a linear electromagnetic actuator may be preferred although a rotary actuator may alternatively be employed.
In any such case the variation in compliance may be controlled by signals generated as an integral of a position demand signal applied to the electromagnetic actuator.
For this purpose it is preferred that the variation in compliance is controlled in dependence on the electrical current required to move the electromagnetic actuator against the load.
The compliant means may be a gas spring and the variation in compliance is achieved by varying the mass of gas contained within a chamber of variable volume.
Such variation may be achieved by controlling valves which allow gas into and/or out from the said chamber.
Although the compliant support may incorporate a fluid actuator, possibly an hydraulic actuator, the working fluid of which can be directed towards or away from the actuator to vary the compliance thereof.
Variation in compliance may be achieved, for example, by the adjustment of pressures in individual gas springs. In one embodiment this is effected in accordance with the time integral of the currents drawn by electromagnetic actuator during motion. By this means the characteristics of the compliant support means are optimised so as to allow the electromagnetic rams to operate within their ratings and to reduce the power consumed by the complete mechanism.
In one embodiment of the present invention incorporating an electromagnetic actuator, the armature and stator of the electrical machine comprise a piston in cylinder device, the piston or armature being shorter in length than the cylinder or stator and being wholly contained within it at all times, the cylinder being closed at both ends by end members, the piston being provided with a rod or thrust element extending through at least one of the end members and being provided with an air seal or air seals to said end members, the armature or piston element being also provided with an air seal whereby to divide the cylinder into two chambers, the armature including first means for producing a magnetic field pattern comprising at least two magnetic poles of opposite polarity and if more than two poles then comprising magnetic fields so arranged as to be of alternating polarity along at least part of the axial length of the armature and the stator being provided with second means for producing a further magnetic field pattern of at least two poles of opposite polarity and if more than two magnetic poles then poles so arranged as to be of alternating polarity along at least part of the axial length of the stator, the second field pattern being so arranged as to interact with the first magnetic field pattern so as to produce an axially-directed force.
Preferably the physical and the electrical parameters of the device are so arranged that-the electrical terminals may be connected to one or more conventional electronic drive units for controlling the phase and amplitude of at least one of the magnetic field patterns so as to cause a desired axially-directed electromagnetic force to be created between the piston and the armature.
Preferably the two chambers, one on each side of the piston and having a volume variable in accordance with the position of the piston within the cylindrical armature, are provided with pipe connections so that the mass of the fluid within them may be controlled. This allows the piston to perform simultaneously as an electromagnetic device and as a double-acting fluid ram.
According to another aspect of the present invention, therefore, there is provided apparatus for controlling relative motion with a plurality of degrees of freedom between a platform and a reference plane, comprising compliant means for supporting the weight of a platform, one or more actuators for applying perturbing forces between the platform and the reference plane, and control means for controlling the or each actuator to move in one direction or the other whereby to displace the platform with respect to the reference plane, characterised in that the compliance of the compliant support means is variable, and there are: provided means for dynamically varying the compliance thereof in dependence on control signals applied thereto.
In embodiments formed as a motion-base there may be three actuators between the motion platform and the fixed reference plane, which may be a fixed part of the mechanism. The actuators have pivots or hinges connecting them to that part of the apparatus defining the fixed reference plane so as to constrain the locus of motion of the actuator within a respective plane. The three planes thus defined intersect along a vertical line joining the centroid of a lower triangle formed by the pivots of the actuators to the centroid of the motion platform triangle formed by the points of connection of the upper ends of the actuators. The upper ends of the actuators have joints providing universal freedom, so that by choice of the three individual actuator lengths the motion platform can be caused to adopt any chosen attitude within reasonable limits of heave, pitch and roll motion. The centre of mass of the load is preferably positioned to lie above and close to the centroid of the motion platform and having a supporting member connected with universal freedom between the centroid of the lower fixed triangle and the centroid of the upper (motion) triangle, the compliance of the supporting member being optimised according to the dimensions of the motion platform.
In a preferred arrangement the upper motion triangle, defined by the ends of the actuators, is smaller in size than the corresponding triangle on the fixed part of the mechanism defining the reference plane, so as to allow actuators of limited stroke to produce acceptable deviations of the motion platform in pitch and roll and to simplify the problems of affixing a capsule to the motion platform.
Preferably the angle between the actuator and the horizontal plane in the straight and level (operating) position of the motion platform is approximately 45xc2x0. The central support may be a simple member or an assembly and may be a metal or plastics spring, a pneumatic ram or a ram in which a liquid acts on a piston in the actuator, the surface of the liquid distant from the piston being pressurised by a gas within a reservoir.
Alternatively the compliant support may be a bellows unit, with the advantage that no universal joints are required at the interfaces of the bellows unit with the fixed and moving platforms, and the collapsed length of the bellows may be less than half its extended length. Moreover, a bellows needs no internal sliding seal like that required for a sliding piston. As another alternative, or in addition, the central support may be formed from a plurality of pressurised gas struts, so arranged as to provide an over-centre toggle action that removes most of the supporting force when the motion platform adopts the minimum height or loading position.
Preferably the actuators are electromagnetic actuators and are designed to have sufficient reserve capability of thrust so as to be able to accommodate practical deviations of the position of the centre of mass of the motion platform from the ideal position close to the exact centroid of the upper motion triangle.
It will be understood that when a motion-imparting system (hereinafter referred to as a motion base) is in operation there is a central, straight and level position to which the capsule must be continually returned. The occupant or occupants of the closed capsule is or are not aware of this continual centering action, which is carefully controlled to blend into the background of the other motions. It takes as much energy to return the capsule to its starting position as it took to move it away in the first place and this energy may be stored in a spring system. Further, because the centre of mass of the payload is invariably above the centroid of the motion platform, there is a significant torque that aids any pitch or roll movement, which must be opposed by means such as spring action. Nevertheless, the restoring forces produced by the spring suspension must not be too great or else they will demand disproportionately large actuator forces to cause the initial exursions.
When attempts have been made to apply the principles of WO93/01577 to a common form of six-ram, six-axis motion platform (known as a Stewart Platform after its designer), a number of difficulties have been encountered. In particular, it has been demonstrated that if the static load exceeds a certain well-defined limit, depending on its height above the centroid of the motion platform, the mechanism has a tendency to xe2x80x9cnose-divexe2x80x9d by a combined forward and pitching motion, from which it cannot recover by electromagnetic forces alone. This defect imposes a strict limit on the payload capacityxe2x80x94and therefore the usefulnessxe2x80x94of the machine.
In another aspect, therefore, the present invention provides a motion-imparting system having six degrees of freedom using six actuators arranged as first described by Stewart, in which the concept described in International Patent Application WO93/01577 namely the weight counterbalancing system is not applied, but in which the forces for supporting the static load are applied in a different, specific and efficient manner, which greatly reduces the electromagnetic power demands on the actuators for any given payload, thus raising the payload limit and improving the dynamic performance of the apparatus.
According to another aspect of the present invention, therefore, there is provided apparatus for controlling relative motion in a plurality of degrees of freedom between a motion platform and a reference plane, comprising means for supporting the weight of the motion platform, one or more actuators for applying intermittent forces between the platform and the reference plane, and control means for controlling the or each actuator whereby to vary the position and/or orientation-of the platform with respect to the reference plane, characterised in that the means for supporting the weight of the motion platform comprise respective compliant support members each associated with a respective said actuator.
In the present invention, in the idealised case of a Stewart platform, three points of the motion platform that lie on a triangle are connected to three corresponding points defining a triangle on the part of the mechanism defining the reference plane by means of six electromagnetic actuators having joints providing universal freedom at the interfaces between the actuators and the fixed platform and similar joints between the actuators and the motion platform, so that by choice of the six individual actuator lengths the motion platform can be moved to adopt a wide range of orientations by motion which may be any one, or any combination of, motions commonly known as heave, surge, sway, yaw, pitch and roll motion. The centre of mass of the load is preferably positioned to lie above and close to the centroid of the motion platform, and the diameter of the motion platform circle preferably has an optimum ratio to the diameter of the fixed reference plane circle. The term xe2x80x9ccirclexe2x80x9d of the platform or plane is here intended to mean the circumscribing circle around the points of attachment at the ends of the actuators to, the platform or the reference plane. Each electromagnetic ram may be associated with an individual spring or be so designed and assembled to the motion base as to act also as the output actuator of an individual spring, the springs being such as to support the weight of the motion platform (and any load thereon) in the central straight and level position.
It is a feature of the present Invention that the spring rates are optimised in relation to the power consumption of the apparatus and the forces exerted by each individual spring are preferably adjustable by a monitoring system so as to reduce to zero the integral of the associated actuator current over a chosen time interval.
Preferably the upper motion circle is smaller in size than the corresponding circle on the fixed part of the mechanism defining the reference plane and the ratio of the two radii is chosen to optimise the power demand. The optimum ratio of base dimensions is close to 1:1.5.
Preferably the actual values of the two radii are chosen such that the angle between the actuators and the horizontal plane when all six actuators are at 50% extension is approximately 45 degrees.
If a gas spring suspension is employed, preferably the ratio of the sealed volume of each gas spring system when the actuator is fully extended to the sealed volume when the actuator is fully retracted is also chosen to minimise the operating power consumption of the apparatus. This optimum ratio of gas spring volumes is considered to be in the vicinity of 1.8.
Preferably means are provided for monitoring the magnitude and direction of the electrical current demand for each actuator and the pressure in each gas spring is arranged to be frequently adjusted during operation in relation thereto, so as to maintain an integrated symmetry of electrical power demand over a chosen period of time.
In embodiments of the present invention, three points of the motion platform may be connected to three corresponding points on the fixed part of the mechanism defining the reference plane by actuators having universal freedom at the connection between the actuators and the reference plane and between the actuators and the motion platform, so that by choice of the three individual ram lengths the motion platform can be moved to adopt any chosen attitude within the limits of movement of the actuators in heave, pitch and roll motion. The centre of mass of the load is preferably positioned to lie above and close to the centroid of the motion platform and preferably has a supporting member connected with universal freedom between the centroid of the reference plane and the centroid of the rotation platforms. In this context it is assumed that the centroid of a platform or plane is the centroid of the triangle defined by the three points of connection to the three (or six) actuators. The spring rate of the support is preferably optimised to the parameters of the platform and the load.
In a preferred embodiment the triangle of actuator connections to the motion platform is smaller in size than the corresponding triangle on the fixed part of the mechanism defining the reference plane, so as to allow actuator rams of limited stroke to produce acceptable deviations of the upper platform in pitch and roll and to simplify the problems of affixing a capsule to the motion platform. Preferably the ratio of the size of the fixed (base) platform defining the reference plane to that of the motion platform is approximately 1.5:1
Preferably the angle between the actuators and the horizontal plane when all three actuators are in the straight and level (operating) position of the motion platform is approximately 45xc2x0.
Preferably the central support is a bellows unit. This has the advantage that the upper and lower ends of the bellows may be fixed directly to the motion platform and to a fixed base defining the reference plane. The collapsed length of the bellows may be less than half its extended length and there is no need for an internal sliding seal. It will be understood that one of the characteristics of a bellows assembly is that in an upright or vertical axis orientation it will permit vertical motion and the upper end of the bellows may be tilted at any pitch or roll angle with respect to the lower end, but it will not readily permit lateral translation (surge or sway) nor will it permit axial rotation (yaw) at all. Thus a bellows assembly may function both as a gas spring unit and as a restraint mechanism.
Preferably the actuators are electromagnetic rams designed to have sufficient reserve capability of thrust so as to be able to accommodate practical deviations of the position of the centre of mass of the motion.platform from the ideal position close to the exact centroid of the upper motion triangle.